1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image using electrophotographic method.
2. Description of the Related Art
In a conventional image forming apparatus using an electrophotographic method, a charging device charges a surface of a cylindrical image bearing member such as a photosensitive member that is rotatably driven by a rotation shaft. The exposure device in the image forming apparatus then exposes the charged photosensitive member to form an electrostatic latent image on the photosensitive member.
The electrostatic latent image formed on the photosensitive member is developed using toner. At a transfer unit, the developed toner image is transferred to a recording medium directly or via an intermediate transfer member. An image is thus formed on the recording medium. The toner remaining on the photosensitive member when forming the image is removed from the surface of the photosensitive member by a cleaning device including a cleaning blade that is in contact with the photosensitive member.
The film thickness of the photosensitive layer on the surface of the photosensitive member is not uniform at each of the positions in a direction of the rotation shaft and a rotational direction of the photosensitive member. This is due to a production error when the photosensitive member is manufactured.
Further, as the cumulative number of times of image formation becomes large, the film on the photosensitive layer may be scraped off due to the contact with the recording medium or the intermediate transfer member at the transfer unit, or the contact with the cleaning blade. Since the amounts of the photosensitive layer that is scraped off are different depending on the positions where the toner is adhered and where the toner is not adhered on the photosensitive member, the film thickness becomes uneven.
As a result, even when the photosensitive member is uniformly charged by the charging device and exposed by a constant exposure amount, the potential on the surface of the photosensitive member becomes uneven. More specifically, sensitivity to voltage and light on the surface of the photosensitive member slightly differs at each position.
In such a case, density unevenness is generated in the formed image. There are techniques as is described below to correct the density unevenness in the image caused by unevenness of a potential characteristic generated on the surface of the photosensitive member.
Japanese Patent Application Laid-Open No. 2005-66827 discusses storing in the image forming apparatus, surface potential data when charging the photosensitive member or surface potential data acquired from the density data of the solid image output to the recording medium.
When the exposure unit exposes the photosensitive member, exposure intensity is adjusted according to an exposed position based on the above-described surface potential data. The in-plane unevenness of the potential characteristic of the photosensitive member is thus compensated by the exposure amount, as is described below.
FIG. 16 illustrates a potential distribution on the surface of the photosensitive member when the photosensitive member in a conventional image forming apparatus is charged and exposed uniformly.
More specifically, FIG. 16 illustrates the unevenness of the potential characteristic on the surface of the photosensitive member, in the image forming apparatus. Such unevenness is generated when the photosensitive member is charged by the charging device and exposed with a constant amount of light to form an image. A main scanning direction of the photosensitive member (i.e., direction parallel to the rotation shaft of the photosensitive member) is indicated in the shorter direction of the distribution. A sub-scanning direction of the photosensitive member (i.e., a direction perpendicular to the main scanning direction, or a rotational direction of the photosensitive member) is indicated in the longer direction. Further, the potential (V) is indicated in the vertical direction.
The unevenness of the potential characteristic of the photosensitive member is caused by a difference of charging sensitivity at each of the positions on the surface of the photosensitive member when the photosensitive member is charged. Moreover, the unevenness is caused by a difference in the potential drop rate at each of the positions on the surface of the photosensitive member when the photosensitive member is exposed.
FIG. 17 illustrates a potential distribution on the photosensitive member when the photosensitive member is uniformly charged and exposed by a constant amount of light. More specifically, FIG. 17 illustrates the potential distribution on the photosensitive member along one line in a direction of the rotation shaft of the photosensitive member (i.e., main scanning direction). The potential is indicated on the vertical axis, and the position on the surface of the photosensitive member in the main scanning direction is indicated on the horizontal axis.
Referring to FIG. 17, after the photosensitive member is charged and exposed, an appropriate potential of the photosensitive member for forming an image may be 50 V. Therefore, the exposure intensity is controlled to compensate the effect of the unevenness of the potential characteristic generated. The exposure intensity is thus increased at a position where the potential is higher than 50 V and decreased at a position where the potential is lower than 50 V when the photosensitive member is uniformly charged and exposed.
The exposure intensity is controlled when the exposure unit scans and exposes the photosensitive member in the main scanning direction and the rotational direction of the photosensitive member (i.e., the sub-scanning direction). As a result, the unevenness of the potential characteristic generated over the entire periphery of the photosensitive member can be corrected.
Further, when the unevenness of the potential characteristic of the photosensitive member is corrected in the sub-scanning direction by changing the exposure intensity, a rotational phase of the photosensitive member is to be managed. The exposure intensity is thus changed according to the rotational phase. An example of a method for managing the rotational phase of the photosensitive member is to use a known home position sensor.
In such a method, the home position sensor detects a home position of the photosensitive member after a predetermined time has passed from when the photosensitive member starts rotating to form the electrostatic latent image until the rotation is stabilized. The exposure intensity in the sub-scanning direction is then changed according to the rotational phase of the photosensitive member thereon.
The unevenness of the potential as described above is to be corrected before forming an image to acquire a high-quality output image. For example, Japanese Patent Application Laid-Open No. 8-130626 discusses an image forming apparatus including a function for printing an image by adding minute dots to the original image. When the print product is copied, the image forming apparatus determines whether the print product can be copied according to a usage restriction expressed by a pattern formed by the added minute dots.
The added minute dots is to be precisely formed into an image and to uniformly reproduce the minute dots in the image surface, so that information indicated by the minute dots can be stably read when the image is copied. Therefore, if the image is formed without correcting the unevenness of the potential characteristic of the photosensitive member that causes unevenness when reproducing the minute dots on the image surface, the minute dots becomes unreadable.
However, there are some images in which correction of the unevenness of the potential characteristic and forming of a high-quality image are not required. In such a case, when the unevenness of the potential characteristic is corrected in the sub-scanning direction of the exposure unit, i.e., the rotational direction of the photosensitive member, by changing the exposure density, the image forming is started in response to the detection of the home position of the photosensitive drum by the home position sensor.
If the control to correct the unevenness of the potential characteristic according to the rotational position of the photosensitive member is not performed, the image forming apparatus can output an image when a preparation time 1801 elapses, as illustrated in FIG. 18.
The preparation time refers to a time period from the time when an image signal is input while the image forming apparatus is in an off or standby state to the time when the rotation speed of the photosensitive drum, the rotation speed of a rotational polygonal mirror, which deflects laser beams to scan the photosensitive drum for exposing the photosensitive drum, and the fixing temperature of a fixing device reach the respective predetermined values to make the image forming apparatus ready for image forming.
On the other hand, when the unevenness of the potential characteristic, which varies according to the rotational phase of the photosensitive drum, is corrected by changing the exposure intensity, the electrostatic latent image cannot be formed on the photosensitive member, even when the rotation is stabilized, until the home position sensor first detects the home position of the photosensitive member (i.e., time 1802 illustrated in FIG. 18).
The time period from the time when the preparation time elapses to the time when the home position sensor first detects the home position of the photosensitive member is equal to the time period used for one rotation of the photosensitive member at a maximum. As a result, a first print output time increases if the unevenness of the potential characteristic in the sub-scanning direction is corrected when forming an image in which high quality is not required. More specifically, the length of time increases between the start of forming an image and the time when the first sheet is discharged, on which an image is formed.